Electric detonators and electric blasting assemblies



April 19, 1955 w, TAYLOR ErAL ELECTRIC DETONATORS AND ELECTRIC BLASTING ASSEMBLIES Filed Feb. 6, 1951 Inventors TAYLOR WILFRID WILLIAM FORSYTH M v M Afiorneys United States Patent G ELECTRIC DETONATURS AND ELECTRIC BLASTING ASSEMBLIES Wilfrid Taylor, Saltcoats, and William Forsyth, Stevenston, Scotland, assignors to Imperial Chemical indus tries Limited, a corporation of Great Britain Application February 6, 1951, Serial No. 209,532

Claims priority, application Great Britain March 14, 1950 Claims. (Cl. 102-48) The present invention relates to improvements in electric detonators of the kind embodying a device for minimising the possibility of the fusehead being accidentally fired by an electrostatic charge or high potential accumulated or induced on the leading wires. The invention also relates to electric blasting assemblies embodying the improved electric detonators.

It is well known that very considerable electrostatic charges can be built up on the fusehead of an electric detonator under unfavourable conditions, and it is known to provide an electrostatic leak from the metal pole pieces of the fusehead to the detonator casing by which such charges can leak away without firing the defiagrating composition constituting the matchhead. Thus it has been proposed to employ in the electric detonator a fusehead the body of which is nearly as broad as the diameter of the metal detonator casing at one end and tapering away towards the opposite end carrying the matchhead, so that any discharge will take place from the broad portion of the fusehead body across the narrow air gap between this end and the detonator casing, in the hope that the discharge will not involve the narrow portion of the fusehead carrying the matchhead. One objection to this method is that it is difiicult to ensure that the fusehead body will not actually touch the detonator casing and thus lead to a short circuit which will prevent the fusehead being fired in the normal manner.

It has also been proposed to provide between a portion of the fusehead or one of the leading wires exposed within the detonator casing a leak consisting of a connection to the detonator wall of markedly greater effectiveness at high than at low potential difierences' espe cially one of the coherer type. Electric detonators of this type, however, are somewhat limited in their applicability, especially as regards series firing, for the resistance of the electrostatic leak must not be too high if it is to function as an electrostatic leak, and in series firing all the detonator casings are normally at or near to earth potential so that as the number of detonators in the circuit increases the tendency towards excessive leakage of the current applied for normal firing purposes and consequent failure to fire the fuseheads is increased.

According to the present invention an electric detonator has its metal casing connected by an electrical conductor to one pole piece and the entire outer surface of said casing provided with a sheath of insulating material.

The said sheath of insulating material is for instance of a rubbery composition and preferably in the form of an adherent coating of a rubbery material. The electrical conductor is preferably a metal strip or a metal wire.

In connecting the improved detonators made in accordance with the invention in series the insulated lead ing wire associated with that side of the pole pieces carrying the said conductor may or may not constitute the leading Wire so associated in the next detonator in the series, and it is necessary that the cables and leading wires outside of the detonator are insulatedthroughout from the earth.

electric detonators in accordance with one form of the invention, showing the electric detonators in longitudinal axial section. 1

In the drawing 1 represents a metal casing of each el'ec-. tric detonator and 2 the longitudinally bored plug of rubberlike material around which the detonator casing is tightly crimped at its upper end, in a series of over-i. folded indentations which have the effect of reducing the apparent diameter at this end. 3 and 4 are respectively the secondary and primary initiating charges of the detonators and 5 are the matchheads of the electric fuseheads whereof 6 represents metallic pole pieces and 7 insulating layer separating the metallic pole pieces from one another. 8 represents a soldered connection adjoining the end stripped of its insulation 12' of the leading wire 11' of the left hand electric detonator and the end 11 stripped of its insulation 12 of the leading wire of the right hand electric detonator to the right hand pole piece 6 of each detonator and also uniting to these the end of a copper wire strip 10 extending under the plug 2 so as to form a good electric connection with the metallic detonator wall 1 of the corresponding detonator. 9 is a soldered joint connecting the end of the other leading wire 11 stripped of its insulation 12 in the case of the left hand electric detonator and the end of the leading wire 11' stripped of its insulation 12 of the right hand electric detonator to the opposite pole piece. 14 and 15 are insulated terminals connected by means of cables 16 and 17 to an exploder 18 the cables 16 and 17 and the leading wire 11' with its insulation 12', have been shown broken in the drawing, since the assembly is in no way restricted to the use of two electric detonators and the cables 16 and 17 are of a length which would correspond with the number of detonators in the circuit and the distance between the furthest two detonators in the series.

When the circuit is completed in the exploder 18 the current flows by way of the cable 16, leading Wire 11, the matchhead 5 of the left hand detonator, and then through the leading Wire 11', and the leading wire 11 of the right hand detonator back through the cable 17. The metal detonator casing 1 is brought by means of the good electrical connection 10 to the same potential as the right hand pole piece 6 of each of the electric detonators, but, owing to the coating 13 of rubberlike material over the surface of each detonator tube exposed to the exterior, there is no short circuit to earth through the detonator casing.

Should an electrostatic charge collect on say the leading wires 11 and 11' for any reason in the course of an atmospheric disturbance or should a sudden high voltage be induced by a lightning strike this will of course raise the pole pieces 6 to a high potential, but owing to the existence of the good electric contact 10 this charge will be effectively dissipated on to the metallic detonator casings 1 and any discharge of this to earth will take place through the coating 13 of rubberlike material, which will result in this being damaged at the point of the discharge but as there will be no direct discharge from the matchhead 5 across the air gap within the detonator, the matchhead 5 will not be fired as a result of the atmospheric disturbance or discharge.

i 3 What we claim is:

1. An improved blasting assembly comprising a pluility of electric detonators, there being in each of said etonators insulated leading wires, a metal casing conected by an electric conductor to one pole piece of a latch-head firing circuit whereby stray static electrical barges are caused to pass harmlessly to the said metal asing, a sheath of insulating material completely sur )unding said casing, said sheath causing the said static lectrical charges to be retained on said metal casing so lat the whole detonator system is at the same potenal until the said static electrical charges are greater ian the breakdown potential of the said sheath in which ase breakdown occurs and the said charges are disharged to earth, and wherein each of said electric detnators is connected in series with a source of electric urrent wherein the cables and leading wires outside the etonators are insulated from the earth.

2. A blasting assembly as claimed in claim 1 where- 1 the sheath of insulating material is of a rubbery comosition.

References Cited in the file of this patent UNITED STATES PATENTS 1,306,510 Allison June 10, 1919 2,086,533 Burrows July 13, 1937 FOREIGN PATENTS 310,364 Great Britain Apr. 22, 1930 OTHER REFERENCES Blasting Accessories, pages 53 and 60; published in 1925 by E. I. 'du Pont de Nemours & Co., Wilmington,

20 Delaware. 

1. AN IMPROVED BLASTING ASSEMBLY COMPRISING A PLURALITY OF ELECTRIC DETONATORS, THERE BEING IN EACH OF SAID DETONATORS INSULATED LEADING WIRES, A METAL CASING CONNECTED BY AN ELECTRIC CONDUCTOR TO ONE POLE PIECE OF A MATCH-HEAD FIRING CIRCUIT WHEREBY STRAY STATIC ELECTRICAL CHARGES ARE CAUSED TO PASS HARMLESSLY TO THE SAID METAL CASING, A SHEATH OF INSULATING MATERIAL COMPLETLY SURROUNDING SAID CASING, SAID SHEATH CAUSING THE SAID STATIC ELECTRICAL CHARGES TO BE RETAINED ON SAID METAL CASING SO THAT THE WHOLE DETONATOR SYSTEM IS AT THE SAME POTENTIAL UNTIL THE SAID STATIC ELECTRICAL CHARGES ARE GREATER THAN THE BREAKDOWN POTENTIAL OF THE SAID SHEATH IN WHICH CASE BREAKDOWN OCCURS AND THE SAID CHARGES ARE DISCHARGED TO EARTH, AND WHEREIN EACH OF SAID ELECTRIC DETONATORS IS CONNECTED IN SERIES WITH A SOURCE OF ELECTRIC CURRENT WHEREIN THE CABLES AND LEADING WIRES OUTSIDE THE DETONATORS ARE INSULATED FROM THE EARTH. 